Achieving optimal rheological behavior in cement slurries is imperative to well completion operations as well as whenever else the use of cement is called for in drilling or hardening. An effective cement system provides isolation between subterranean zones. To effectively utilize cement in a well operation, the cement slurry, upon mixing with water and necessary additives, fillers, etc., must exhibit fluid behavior that allows it to be pumped efficiently downhole without hardening, settling, or damaging equipment and be placed in the desired location where it will set into a hardened material.
Because the cement slurry is a suspension, the settling of solids is one problem often encountered with cement formulations. Settling may result in packing off in equipment lines at the surface and may result in a non-uniform cement composition when in a static position after placement downhole. Settling problems are often caused by the cement slurry having a low viscosity that cannot adequately suspend solids. To counter settling, viscosifiers or suspension aids are often mixed into slurry formulations as additives. Such additives, however, must be carefully formulated and subsequently added to the cement slurry so as not to reach a viscosity too high for working operations. In addition, such additives should not overly adversely affect the strength of the final product or inhibit the chemical reaction in a manner that cannot be controlled or predicted.
In addition to settling, another problem arises when pre-mature hydration events occur that lead to gelation of cement slurries. Gelation increases the slurry viscosity, rendering the slurry far less workable and less fit for pumping and placement. To prevent slurry gelation, cement retarders are often added to the slurry formula to suppress hydration reactions that lead to gels. Cement hydration retarders, however, impose a significant influence on set time and, therefore, must be carefully formulated. Dispersing additives may also be added to mitigate gel formation through a mechanism resulting in the repulsion of cement particles and preventing aggregation within the cement slurry. Dispersants must also be added with care to avoid over-dispersion, which can re-introduce the aforementioned issue of solids settling.
When both settling of solids and gelation occur, the slurry may become unrecoverable even by addition of powerful dispersants. Concurrent settling and gelation in a slurry gives rise to a material that cannot be used operationally, and is likely to damage equipment if trapped in deposition lines and/or pumps. There is, therefore, a need to develop the means to combat settling and gelation to obtain optimal cement slurry rheology.